Macworld Conference & Expo 2009 was the last time a (non-developer) member of the public could attend an Apple keynote.
It was also the last time an Apple keynote was scheduled a year in advance with no way for Apple to postpone if circumstances change. From here on out, Apple could schedule keynotes on demand, as needed. If they had nothing to announce with a big splash for seven months (like they did from November 2012 to May 2013), they could hold their fire.
Macworld continued without Apple's participation for five more years, getting renamed to Macworld/iWorld for the last three years before it was discontinued. For fun, let's see what Apple would have announced if it still gave a keynote at each one.
Macworld 2010 would have been the iPad show, of course. Apple had its own press-only keynote just two weeks before the expo in February.
Apple did not hold an event anywhere near Macworld 2011 in January. However Tim Cook did appear on stage at the Verizon event that expanded the iPhone's exclusivity in the United States to a second carrier just a few weeks earlier. Apple released new MacBook Pro's with Thunderbolt technology the next month in February. Throw in a retail update, and you've got yourself a keynote. Who knows, if Apple was still doing Macworld, maybe they would have done everything in their power to get the iPad 2 ready a little bit earlier.
In 2012, Apple had its own Education event in New York City just a week before the expo, so clearly that could have been incorporated into the Macworld keynote. Mac OS X Mountain Lion was shown to the press and developers in early February, so that might have made the cut too.
Apple had radio silence for the first five months of 2013, until WWDC in June. There was not even a spec bump for a notebook during this time. But the redesigned iMacs with laminated displays released the previous Fall were really slow in getting out the door. The 21.5" was announced to arrive in November, and ended up shipping November 30. The 27" was promised for December, but only a lucky few got theirs before the new year. In retrospect, Tim Cook wishes they had just delayed the iMac until January. If they had attended the expo in 2013, they would have been forced to anyways.
A similar picture emerged the following year, with Apple not holding any keynotes for the first five months of 2014, but releasing the brand new cylindrical Mac Pro the December before. They would have had to hold off on previewing it at WWDC 2013, but it would pretty much be the only choice for Macworld/iWorld 2014.
The expo probably wouldn't have been cancelled if Apple had continued its participation. Demand for WWDC tickets during this time has increased to such a degree that a lottery had to be implemented, and lineups start forming around the block at 4am.
Demand for tickets to a general admission keynote today would be in the stratosphere. It's another reason they don't do it anymore.
But it sure would be fun if they did it again.